A fuel cell system for generating power by utilizing reform-subject fuel such as hydrocarbon or alcohols comprises a reformer for generating anode gas containing hydrogen from the reform-subject fuel, a hydrogen separation membrane device for extracting high-purity hydrogen from the anode gas, and a fuel cell for protonating the hydrogen and causing it to react with oxygen so that power may be generated. This reformer generates the anode gas by conducting, for example, water vapor reformative reaction by use of reform-subject fuel and water, and oxidative reaction by use of reform-subject fuel and oxygen, etc. Further, the hydrogen separation membrane device is provided with hydrogen separation membrane comprised of palladium, etc., which has such properties that only hydrogen may pass through it. Further, the fuel cell has an anode flow channel to which hydrogen is supplied that has passed through the hydrogen separation membrane, a cathode flow channel to which air, etc. is supplied, and a proton conductor (electrolyte) arranged between these flow channels.
In this fuel cell system, hydrogen supplied to the anode flow channel is protonated and permitted to pass through the proton conductor so that this protonated hydrogen (hereinafter referred to as hydrogen proton) may react with oxygen in the air in the cathode flow channel, to generate power while generating water. Such a fuel cell system is exemplified in Patent document 1.
It is to be noted that cathode offgas released from the cathode flow channel contains water generated by the reaction between the hydrogen proton and oxygen, oxygen, etc. left unused in the reaction of the hydrogen proton. Therefore, Patent document 1 utilizes the cathode offgas in water vapor reformative reaction and oxidative reaction in the reformer.
In Patent document 1, a rate of oxidative reaction in the reformer is controlled by adjusting a quantity of oxygen contained in cathode offgas to be sent to the reformer, and also a rate of water vapor reformative reaction in the reformer is controlled by adjusting a quantity of water vapor in cathode offgas to be sent to the reformer. With this, in Patent document 1, mounting a vaporizer conventionally required in the reformer can be eliminated, thereby the system can be downsized.
However, Patent document 1 discloses no specific method for adjusting the quantities of oxygen and water vapor.
Further, to generate power in the fuel cell system, it is necessary to appropriately keep O/C, which is a proportion of a quantity of oxygen O against a quantity of carbon C supplied to the reformer to adjust the quantity of oxidative reaction performed in the reformer. However, Patent document 1 discloses no countermeasures to be taken if a residual oxygen quantity in the reaction between hydrogen proton and oxygen in the cathode flow channel in the fuel cell increases or decreases due to, for example, fluctuations of power generation condition in the fuel cell.
Further, the method disclosed in Patent document 1 adjusts a quantity of oxygen O and a quantity of water S which are supplied to the reformer with reference to a quantity of carbon C which is supplied from a fuel pump to the reformer, to control O/C and S/C. Therefore, to control O/C, an air release valve is required which releases some of cathode offgas or a control valve is required to limit passage of the cathode offgas. Further, to control S/C, a water vapor generation portion, etc. is required which adjusts a quantity of water contained in the cathode offgas. These may complicate the fuel cell system.    Patent document 1: JP 2000-195534 Unexamined Patent Publication (Kokai)